Many state of the art optical waveguides rely on the ability of the wave-guiding structure to transmit a precise optical signal that is free from interference. This interference can come from external electromagnetic sources (external electromagnetic interference (EMI)), from nearby electro-optical circuits in the same device (cross-talk), or from spurious signals introduced by auxiliary elements of the waveguide itself (parasitic mode propagation). External EMI can be eliminated with proper shielding of the optical waveguide device, and cross-talk can be minimized by isolating elements in the waveguide device.
Parasitic mode propagation, which arises when a waveguide propagates a mode of electromagnetic radiation other than the intended mode(s) as an unforeseen consequence of the materials, methods, and/or geometrical parameters used in the construction of the waveguide, and is especially difficult to minimize because it often arises from the presence of elements of the waveguide that are critical to its operation, especially in military applications. A common cause of parasitic propagation is reflection of weak optical signals from metallic surfaces near the waveguide. These conductive surfaces are an essential element in active electro-optical devices, as electric voltages applied between them are typically the means by which optical signals are modulated. In state of the art waveguide devices, these surfaces are often moved as far away from the waveguide as practical and constructed with the minimum practical size, in order to minimize the effects of parasitic mode propagation. However, these remedies both result in significantly reduced waveguide performance.
Another means of minimizing parasitic mode propagation would be to deposit an anti-reflective coating on the metal electrodes. In order not to significantly impair waveguide performance, the coating material would ideally need to be electrically conductive. The difficulty with highly conductive materials; however, is that they tend to be highly reflective themselves. It is possible to reduce this effect using interference phenomena, but a more effective approach is to use a conductive material with an especially high absorbance at the wavelength of light for which the waveguide has been constructed. Platinum black is known to have such properties and is used as a coating on platinum electrodes to catalyze the reduction of hydronium ion to hydrogen and water (normal hydrogen electrode or NHE).
A major difficulty to overcome would be the need to pattern such a coating to conform to the metal surfaces in the waveguide. Most current anti-reflective coatings are applied as an undefined single or multiple layer stack on a flat surface. While in theory it might be possible to use lithographic etching techniques to pattern the anti-reflective layer to match a previously defined pattern in the substrate, the processing required would add significant time and expense to the production of precision optical waveguides, and in many circumstances this would prohibit the use of such techniques.
Thus, the development of an anti-reflective coating for patterned metal surfaces that is compatible with precision optical waveguide technology and requires no subsequent processing to define a matched pattern in the coating after deposition represents a significant, novel, and practical advance in optical waveguide and related technologies in which patterned anti-reflective coatings are required.
There presently is a need for an antireflective coating which functions as a light absorbing/scattering layer on gold electrodes.